The present invention relates to exhaust gas purification catalysts.
Conventional exhaust gas purification catalysts have a problem in which agglomeration of catalytic metal particles degrades the performance of the catalysts. The agglomeration of catalytic metal particles occurs when a catalyst is exposed to high-temperature exhaust gas. For example, the temperature of an exhaust gas purification catalyst directly coupled to an exhaust manifold of an automobile engine reaches as high as about 1100° C. in some cases. Even if dispersed catalytic metal particles are carried on a support member having a large specific surface area, such as activated alumina, these catalytic metal particles inevitably gradually agglomerate. Conventional catalysts include a relatively large amount of catalytic metal in order to obtain a certain level of catalytic performance even with agglomeration of the catalytic metal. However, precious metals, such as Pt, Pd, and Rh, which are generally employed as catalytic metals are expensive, and resources of such rare metals have been sought in recent years.
To solve the foregoing problem, catalytic metal is not only carried on the surface of, for example, activated alumina, but also dissolved in a CeZr-based mixed oxide (composite oxide) serving as an oxygen storage/release material which stores and releases oxygen according to a variation of the air-fuel ratio of exhaust gas. When catalytic metal is dissolved in this CeZr-based mixed oxide, the oxygen storage/release capacity of the CeZr-based mixed oxide is greatly increased. Accordingly, if the CeZr-based mixed oxide in which catalytic metal is dissolved is used as a three-way catalyst and the air-fuel ratio of exhaust gas is repeatedly changed between lean and rich with respect to the stoichiometric air-fuel ratio, excellent exhaust-gas-purification performance can be obtained even with a small amount of the catalytic metal.
For example, Japanese Patent Publication No. 2005-161143 shows that a precious metal catalyst is placed between atoms, at crystal lattice points, and/or at oxygen defect sites of a CeZr-based mixed oxide, and is carried on the surface of the CeZr-based mixed oxide. Japanese Patent Publication No. 2006-334490 shows that CeZr-based mixed oxide particles containing catalytic metal is carried on the surfaces of CeZr-based mixed oxide particles containing no catalytic metal, and that in the CeZr-based mixed oxide particles containing catalytic metal, the catalytic metal is placed at least one of at and between crystal lattice points of the CeZr-based mixed oxide and is partially exposed at the surfaces of the CeZr-based mixed oxide particles. The technique of Japanese Patent Publication No. 2007-31192 is directed to a ceria-zirconia solid solution sol for cleaning an automobile exhaust gas, and shows that the ceria-zirconia solid solution sol has an average particle diameter of 5 to 100 nm